1. Field of the Invention
The present invention relates to an ion implantation system and more particularly, an improvement of the suppressor electrode for the Faraday cup.
2. Description of the Related Art
The ion implantation technique is a manner of accelerating impurity ions generated in the ion source device by high electric field and introducing impurities into an object such as the semiconductor wafer, which is to be processed, while using the kinetic energy of the impurity ions accelerated. This manner is quite effective in that the total of the impurities introduced into the object can be accurately measured as an amount of charges.
The structure of a typical ion implantation system for implanting impurities into the semiconductor wafer is as shown in FIG. 1. According to the typical ion implantation system having such structure as shown in FIG. 1, materials such as gases are made into plasma in an ion source device 90, and desired positive ions are drawn out from this plasma, accelerated and then shot or radiated to a wafer W. Positive ions in the plasma are electrically drawn out by an extraction electrode 91 and spectroscopically analyzed by a mass spectrometer 92 to separate the above-mentioned positive ions from the positive ions thus drawn out. Further, this ion separation is completely carried out by a separation slit 93 and the desired positive ions thus separated are accelerated to final energy through an accelerating tube 94.
A Faraday cup 95 is located just before the wafer W to shut up therein secondary electrons and ions generated when ions are implanted into the surface of the wafer W not to cause them to flow out and to accurately measure the amount of ions implanted. A suppressor electrode 96 is further located before an inlet of the cup 95 not to cause the secondary electrons to come out of the cup 95. Voltage of -2 KV, for example, is added from a power source E to the suppressor electrode 96. A plasma generating chamber 97 is located on one side of the cup 95 to neutralize positive charges stored on the surface of the wafer at the time when ion beam is shot to the wafer.
The ion implantation system having the above-described structure is disclosed in U.S. Pat. Nos. 5,089,710 and 5,148,039, for example.
In the case of this ion implantation system, an insulating film such as SiO.sub.2, Al.sub.2 O.sub.3, P.sub.2 O.sub.5 or As.sub.2 O.sub.3, which is a byproduct created at the ion implantation process, is formed on the surface of the suppressor electrode 96 as the suppressor electrode 96 is repeatedly used. In addition, charged particles included in plasma generated in the plasma generating chamber 97 or caused when the ion beam collides with remaining gases are present in the vicinity of the electrode 96. Positive charged particles of them are attracted by the suppressor electrode 96. Because of the action of the above-mentioned insulating films formed on the surface of the electrode 96, however, the positive charged particles are not taken into the electrode 96 but stored on the top of the insulating films.
FIG. 2 schematically shows the state of the positive charged particles stored, together with an equivalent circuit. Symbols DI, C and V in FIG. 2 represent the layer of insulating films, the capacity of the insulating films, and suppressing voltage, respectively. When ion beam is shot to the surface of the wafer W, the wafer itself, a wafer supporting disk made of aluminum and a photo-resist film formed on the wafer W are sputtered by the ion beam. Particles thus generated by sputtering are scattered, combined with ions to be implanted and stuck to and stored on the surface of the suppressor electrode 96. It is supposed that the insulating film layer DI is formed on the surface of the suppressor electrode 96 in this manner.
When the amount of charges of the positive charged particles stored on the surface of the suppressor electrode 96 exceeds the break down voltage of the insulating film layer DI, discharge is caused between the surface of the insulating film layer DI and the electrode 96 and current is thus allowed to rapidly flow to this discharge-caused part. Because the electrode 96 is a conductor, electrons which correspond to the current are supplied from the voltage source E to the part and large instant heat energy is thus caused there by the discharge. As the result, a part of the insulating film layer DI on the electrode 96 is scattered like a burst together with a part of the electrode 96 by the impact of the discharge, as shown in FIG. 3, and particles are thus caused.
Further, semiconductor devices are more and more highly integrated and those cases where impurities must be implanted into the wafer only by an extremely small depth are therefore increased. According to the conventional ion implantation system, voltage in the accelerating tube 94 is set, in these cases, reverse to that at the time of acceleration to decelerate the ion beam. According to this manner, however, the depth into which impurities are implanted cannot be made uniform. Needs asked by the devices are still left not satisfied accordingly.
Still further, vapor of a matter such as tungsten of which the filament is made and particles caused when a matter such as molybdenum of which inner walls of the plasma generating chamber 97 are made is sputtered by plasma are present, although quite small in amount, in the plasma generating chamber 97. Atoms of these heavy metals fly out outside the chamber 97 to thereby contaminate the surface of the wafer W.